Studies of food, water and brain-stimulation rewards have provided evidence that central dopaminergic (DA) substrates represent an important link in the neurobiology of positive reinforcement. Since abused drugs are presumed to act via a central activation of the brain's natural reward mechanism(s) it has been suggested that these DA pathways also mediate the reinforcing properties of self-administered drugs. Controversy exists, however, over whether or not such DA systems underlie the reward produced by all positive reinforcers or whether various independent systems exist in parallel within the CNS. The proposed research was devised to test the hypothesis that central DA neurons mediate the reinforcing properties of two major classes of abused drugs - the psychomotor stimulants (amphetamine, cocaine), and the opiate agonists (morphine and heroin). the effects of peripheral and central DA receptor antagonists on drug reward shall be examined in two novel behavioral paradigms: a Partial Reinforcement Extinction Test and an Incentive Motivation test. Both tests have the important advantage of assessing reward changes in animals that are no longer drugged at the time of testing. The proposed research should a) provide information relevant to our understanding of human stimulant and opiate drug abuse; and b) help elucidate the role of central DA neurons in the neural basis of drug reward.